The present invention concerns a novel, communication-transmitting optical fiber which has high stability to nuclear radiation.
Optical fibers for communication must be distinguished by low transmission losses of less than 20 db/km, preferably of less than 10 db/km, and by a low pulse dispersion of less than 6 ns/km, preferably less than 4 ns/km, if they are to be suitable for the transmission of information over more than 500 m at an adequate bandwidth of more than 100 megahertz.
Such fibers have been developed recently, for example, those disclosed in DAS No. 2,546,162 (British patent application No. 42779/76; U.S. patent application Ser. No. 732,197 now U.S. Pat. No. 4,148,621), whose disclosures are incorporated by reference herein. With reference to FIG. 1a, these fibers have a light-conducting core 1 made of a multicomponent glass surrounded by an outer sheath made of a silica glass 3. They are made by coating the inside of a silica tube with a series of multicomponent glass layers, each successive layer having a higher index of refraction (n) than the previous one, so that the overall index of refraction profile for the core is that of a parabola (see FIG. 1b). The coated tube is subsequently collapsed and drawn to form the fiber. Preferably, the layers are deposited using the Schott High Temperature CVD Process (see copending U.S. application Ser. No. 941,692, filed on Sept. 12, 1978.
A preferred optical fiber has an intermediate cladding, i.e., an optical insulation zone 2, between the multicomponent glass core and the exterior silica glass sheath. (See, for example, British Pat. No. 1,436,605; J. Am. Ceramic Soc. 58 (5-6), 261 (1975); U.S. Pat. No. 3,963,469.) This optical insulation zone is made of a borosilicate glass (B.sub.2 O.sub.3 -SiO.sub.2) cladding, which can also be deposited by CVD techniques.
Although such fibers have excellent properties under normal usage, after being exposed to the effects of nuclear radiation, their properties (attenuation of transmission; dispersion of pulsewidth, etc.) significantly deteriorate. Consequently, the use of communication fibers having such pronouncedly good properties generally becomes impossible if higher doses of such radiation are impinged on such fibers. For example, loads of 10.sup.4 rad make the use of such fibers impossible, despite the fact that otherwise they would actually be more suitable for interference-free transmission than electromagnetic or electric transmission lines. This deterioration in performance is especially due to the fact that such radiation loads very strongly damage the core glass and the optically insulating borosilicate glass zones, thus leading to high transmission losses due to the irradiation.